The Cambrian Explosion is considered to be an 'explosion' because of the rapidity with which it took place when compared to the entire history of life on Earth. For 3.8 billion years prior to the Cambrian Explosion, life had existed on Earth as little more than bacteria, plankton and multicellular algae. Then in a span of about 30 to 40 million years, most of the basic body plans of animals were established. In fact, most of the body plans appeared during a flash of just 5 to 10 million years. When compared to the entirety the 3.8-billion-year history of life on Earth, this was indeed an explosion.
The most compelling evidence for the Cambrian Explosion was unearthed from the Burgess Shale Formation, a fossil site nestled amongst the Canadian Rockies of British Columbia. The fossils and sediments of the Burgess Shale Formation were deposited during the middle Cambrian when the blossoming of animal diversity was in full swing. The Burgess Shale preserved animals that included brachiopods, trilobites, molluscs, echinoderms, and numerous extinct animal groups.